規模比恐龍滅絕還大的遠古死亡事件
By Danielle
Torrent Tucker
氧氣大量進入大氣的時候遠古生命也變得更加繁盛。但經過數億年之後地球的氧氣減少，造成了比恐龍滅絕更加慘重的死亡事件。
圖中為加拿大哈德遜灣貝爾徹群島的岩石。史丹佛大學的博士候選人Malcolm
Hodgskiss從裡面採集了年代為20.2億至18.7億年前的重晶石樣品。圖片來源：Malcolm
Hodgskiss
科學家從加拿大數十億年形成的岩石中得出的線索，揭露了一場前所未聞的大滅絕事件。6500萬年前的恐龍大滅絕事件發生時，地球將近四分之三的動植物種類就此消失。但這場事件消失的物種數目可能還要更多。
這些滅亡的物種並非可以四處覓食的動物，而是十分微小的微生物。它們逐漸改變了地球大氣，最後形成了可以讓大型動物繁榮生長的良好條件。
這篇新研究發表在《美國國家科學院院刊》(Proceedings
of the National Academy of Sciences)，共同作者Malcolm
Hodgskiss表示：「結果顯示就算地球上的生物都是由微生物組成的，還是會發生可以視為大規模滅絕的事件，雖然它們會以不同於化石紀錄的方法保存下來。」
無法看見的線索
由於這段時間早在複雜生命出現之前，所以研究人員想要得知20億年有什麼樣的生物活著，無法單憑挖掘化石來解答，甚至要從泥巴與岩石中找到線索並加以分析都相當困難。
研究團隊轉而運用重晶石這種礦物。他們從加拿大哈德遜灣貝爾徹群島採集到的重晶石，封存了大氣氧含量如何變化的紀錄。大約20.5億年前地球生物圈(活著的生物在地球佔據的範圍)經歷了一場重大變化，使得生物大量減少。而重晶石樣品顯示這場劇變或許跟氧含量下降有關。
「這種地球化學訊號能夠保存下來相當令人驚訝。」Hodgskiss表示，「這些重晶石十分不尋常的地方是，它們顯然經過了一段相當複雜的歷史。」
研究過往歷史中地球的生產力，可以讓我們一窺生物自從出現之後的整體狀況可能為何――此外，也能讓我們了解觀察太陽系以外的行星大氣得到的結果。
史丹佛大學地質科學系的助理教授Erik
Sperling並未參與此研究。他說：「我們在研究地球歷史的時候，長久以來有個相當大的謎題是生物圈的大小隨著地質時間經過有何變化。這項新的代用指標描繪出生物圈、大氣中的氧氣以及二氧化碳含量三者之間有何關聯。」
從生物的角度出發
生命的繁榮跟大氣氧含量有關，讓研究人員有了支持「氧過量」假說的新證據。這項假說認為遠古微生物行使的光合作用加上岩石風化造成大氣出現了大量氧氣，但是這些產氧生物隨後耗盡了海洋中的養分，數量因而減少，連帶讓大氣氧含量降下來。這種情況跟我們現在所知的地球不一樣：由於氧氣產生和消耗的速率達到平衡，因此目前大氣氧含量是穩定的。研究人員測量重晶石的氧、硫、鋇三種同位素得到的結果，支持了氧過量假說。
研究結果顯示了氧含量超過或低於地球的容量時對生物造成的重大影響，這有助於科學家更加精確地估計氧過量的規模有多大。
「某些對於當時氧含量的估計結果，需要海洋裡面住著的微生物數量實在是太多了。」魏茨曼科學研究所暨普林斯頓大學的博士後研究員Peter
Crockford是研究共同作者，他說：「因此我們現在開始能從生物的角度，來把當時地球大氣的成分縮限在更小的範圍。」
Ancient die-off greater than the
dinosaur extinction
When significant oxygen entered the
atmosphere, ancient life multiplied. But after a few hundred million years,
Earth’s oxygen plummeted, resulting in a die-off likely greater than the
extinction of the dinosaurs.
Clues from Canadian rocks formed billions of year ago
reveal a previously unknown loss of life even greater than that of the mass
extinction of the dinosaurs 65 million years ago, when Earth lost nearly
three-quarters of its plant and animal species.
Rather than prowling animals, this die-off involved
miniscule microorganisms that shaped the Earth’s atmosphere and ultimately
paved the way for those larger animals to thrive.
“This shows that even when biology on Earth is
comprised entirely of microbes, you can still have what could be considered an
enormous die-off event that otherwise is not recorded in the fossil record,”
said Malcolm Hodgskiss, co-lead author of a new study published in Proceedings of the National Academy of
Sciences.
Invisible clues
Because this time period preceded complex life,
researchers cannot simply dig up fossils to learn what was living 2 billion
years ago. Even clues left behind in mud and rocks can be difficult to uncover
and analyze.
Instead, the group turned to barite, a mineral
collected from the Belcher Islands in Hudson Bay, Canada, that encapsulates a
record of oxygen in the atmosphere. Those samples revealed that Earth
experienced huge changes to its biosphere – the part of the planet occupied by
living organisms – ending with an enormous drop in life approximately 2.05
billion years ago that may also be linked to declining oxygen levels.
“The fact that this geochemical signature was
preserved was very surprising,” Hodgskiss said. “What was especially unusual
about these barites is that they clearly had a complex history.”
Looking at the Earth’s productivity through ancient
history provides a glimpse into how life is likely to behave over its entire
existence – in addition to informing observations of atmospheres on planets
outside our solar system.
“The size of the biosphere through geologic time has
always been one of our biggest questions in studying the history of the Earth,”
said Erik Sperling, an assistant professor of geological sciences at Stanford
who was not involved with the study. “This new proxy demonstrates how
interlinked the biosphere and levels of oxygen and carbon dioxide in the
atmosphere are.”
Biological angle
This relationship between the proliferation of life
and atmospheric oxygen has given researchers new evidence of the hypothesized
“oxygen overshoot.” According to this theory, photosynthesis from ancient
microorganisms and the weathering of rocks created a huge amount of oxygen in
the atmosphere that later waned as oxygen-emitting organisms exhausted their
nutrient supply in the ocean and became less abundant. This situation is in
contrast to the stable atmosphere we know on Earth today, where the oxygen
created and consumed balances out. The researchers’ measurements of oxygen,
sulfur and barium isotopes in barite support this oxygen overshoot hypothesis.
The research helps scientists hone their estimates of
the size of the oxygen overshoot by revealing the significant biological
consequences of oxygen levels above or below the capacity of the planet.
“Some of these oxygen estimates likely require too
many microorganisms living in the ocean in Earth’s past,” said co-lead author
Peter Crockford, a postdoctoral researcher at the Weizmann Institute of Science
and Princeton University. “So we can now start to narrow in on what the
composition of the atmosphere could have been through this biological angle.”
原始論文：Malcolm S. W. Hodgskiss, Peter W. Crockford,
Yongbo Peng, Boswell A. Wing, Tristan J. Horner. A productivity collapse to end
Earth’s Great Oxidation. Proceedings of the National Academy of Sciences,
2019; 116 (35): 17207 DOI: 10.1073/pnas.1900325116
引用自：Stanford's School of Earth, Energy &
Environmental Sciences. "Ancient die-off greater than the dinosaur
extinction."
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